The invention applies generally to methods and apparatus for determining the direction of a source of radio signals received by a four arm log spiral antenna. More particularly, the invention applies to a phase comparison method and apparatus for processing signals from a four arm log spiral antenna receiving signals from a radio source to determine the rectangular coordinate direction angles of the radio source relative to the antenna.
Many types of monopulse receivers are used to process received signals from a four arm log spiral antenna system with dual mode excitation. These known receivers use either amplitude comparison or a combination of amplitude and phase comparison monopulse, such as, for example, described in U.S. Pat. No. 3,863,259, issued Jan. 28, 1975 to Boughnou et al.
In known amplitude comparison systems, linear IF amplifiers are used for signal amplification in the .SIGMA. and .DELTA. channels, which must track in both amplitude and phase. Typical IF channel gains (including video) are approximately 100 dB and depending on the application, typical amplitude and phase tracking are required to be within .+-.2 dB and .+-.20.degree., respectively. Also, the dynamic range of the received signals are normally sufficiently large that AGC is required for channel linearity.
A phase comparison method and apparatus for use with a four arm log spiral antenna is highly desirable, in that in such a method or apparatus, linear IF amplifiers can be replaced with limiting amplifiers. Also, such a method, using only phase comparison, does not require AGC, log amplifiers, or square law detectors for the conversion from polar to rectangular direction angles, as do present methods and apparatus utilizing amplitude comparison.